1) Field of the Invention
The present invention relates to a drive control method and a drive control device that use a pulse motor, a belt apparatus, an image forming apparatus, an image reading apparatus, and a computer product.
2) Description of the Related Art
Conventionally, to perform position control of a rotating body, an encoder is arranged on an axis of the rotating body and the rotating body is controlled based on feedback from the encoder. The rotating body may a cylindrical drum, a circular plate, or a belt laid over at least two shafts. The position control here means angular displacement control or displacement control.
However, in the conventional technology, eccentricity of the shaft to which the encoder is attached, eccentricity of an attachment position of the encoder with respect to the shaft, and the like causes wrong position control. One approach to overcome this problem is to fix a scale directly on the surface of the rotating body and read the scale with a reflective photo-sensor (hereinafter, “sensor”).
However, the scale may become damaged during operation and may produce incorrect readings which leads to incorrect position control.
Moreover, there is always a distinct boundary (hereinafter, “joint”) between the rotating body and the scale, which also produces incorrect readings. Japanese Patent Application Laid-Open No. 2002-136164 discloses a countermeasure. According to this publication, when a pulse does not reach within a pre-set time, it is judged that it is a joint that has produced the incorrect readings. Moreover, rotation velocity of the rotating body is corrected based on the judgment.
Japanese Patent Application Laid-Open No. H9-229957 discloses providing reference marks on a rotating drive shaft of the rotating body and detecting those reference marks. Velocity control and position control of the rotating body is performed based on result of detection of the reference marks. However, eccentricity of the drive shaft, errors in the positions of the reference marks, and the like produces incorrect detection and results in inaccurate feedback control. As a countermeasure, there is known a method of providing reference marks directly on the surface of the rotating body.
Most of the existing driving control apparatuses that employ pulse motors use a feedback control loop for angular displacement, which makes accurate control of the pulse motor possible. However, this drive system has a drawback in that, for example, when a detector breaks down or, in particular, when there is a wrong output in a detection signal in, for example, a joint part of a scale at the time when a surface displacement sensor is used, accurate control cannot be performed.
As a countermeasure, Japanese Patent Application Laid-Open No. 2002-136164 discloses providing means for interrupting rotating velocity correction in a velocity control apparatus of the rotating body in which a feedback system using a pulse motor is established. However, with only a velocity control system, it is impossible to cope with a highly accurate position control system.
In the method described in Japanese Patent Application Laid-Open No. 2002-136164 a timer is required for judging the joint. This causes an increase in cost due to an increase in the number of components.
In treatment of the joint, there is also a method of using plural sensors and, when one sensor is judged as abnormal, switching the sensor to another sensor. However, two or more sensors are required in this method, and an increase in cost is also unavoidable.
In addition, the method described in Japanese Patent Application Laid-Open No. 2002-136164 is based on an idea of providing two sensors preliminarily assuming that accuracy is not guaranteed by detection with one sensor. Thus, an increase in cost by an increase in the number of sensors is unavoidable. In addition, when abnormality has occurs with time in an optical pattern (scale) that is a detection object, the number of sensors is meaningless, and suspension and the like of an apparatus operation are unavoidable.